While the instance has a similar layout to the Obsidian Sanctum philosophically, the three sub-bosses (Saviana Ragefire, Baltharus the Warborn, and General Zarithrian) will each be own encounter all their own. The "drakes-up" functionality of Sartharion will not carry over into the Halion encounter.

Heroic difficulty is likely to be toggled in the same way that heroic difficulties are handled in ICC (a UI toggle).

Saviana, Baltharus, and Zarithrian will each drop an Emblem of Frost, but probably not special loot.

Expect loot in the instance to be "about the same item level" as the Lich King items on their respective difficulties.

The loot from Halion will be offset pieces as well as trinkets and other items "of that nature." So, basically, Sartharion loot.

The instance is intended to be a prelude to Cataclysm and should provide some advancement for the new expansion's storyline before the 4.0 patch, drawing on Deathwing's desire to weaken the Wyrmrest Accord prior to his emergence from Deepholm.

Arthas is still considered the final boss of the expansion -- this is merely a complementary raid, like Onyxia with ToC, with ties to Cataclysm.

Sounds like about what we were expecting. Keep an eye on WoW.com for more information on the Ruby Sanctum as it comes in, likely starting with the upcoming PTR.